Preload indicting washer assemblies are used with fastening devices such as, for example, nuts and bolts, for critical applications where it is important to know a predetermined compression value applied by the fastening devices or an axial tension in a bolt, rod, etc. Examples of important applications for such assemblies may include the fastening of certain critical joints of aircraft, etc.
The use of torque wrenches do not always provide consistent indications of the compression or load actually applied by the fasteners. That is, even when the same torque (as read by a torque wrench) is applied to a plurality of fasteners of the same fastener type and size, differing preloads are actually experienced on the fasteners due to tolerance variations in the manufacture of the fasteners, including small variations in fastener dimensions, materials and coatings, as well as variations in any lubrication that may be applied. It is believed that under some circumstances, the application of the same torque as read by a torque wrench to a plurality of bolts supplied under the same part number, can result in a variation of 25% or more in the load actually applied by the bolts. Thus preload indicating washer assemblies can be used with application-critical fasteners to provide a more accurate indication of the actual compression applied by the fasteners on their associated joints. Additionally, these assemblies can be used to provide an indication of compression in joint locations that are not accessible by torque wrenches.
Known preload indicating washer assemblies function by a plastic deformation of an inner ring member to “indicate” when the proper bolt load is reached. That is, the inner ring member is concentrically mated with an outer ring member having a shorter axial height than the inner ring member. The inner and outer ring members are sandwiched between two, standard washers. This assembly is mated with a bolt or other fastener so that the shorter, outer ring member can be made to manually rotate around both the taller inner ring member as well as the fastener when no compression, or when a compression below the rated load, is applied.
In operation, the inner ring member is subjected to compression applied by the fastener, such as a bolt, as two or more components of a structure are being assembled. When the desired, predetermined compression is reached, the inner ring member is plastically deformed so that the two standard washers abut each end of the outer ring member with sufficient force that the outer ring member is no longer able to rotate around the inner member or the fastener. Thus by manually attempting to rotate the outer ring member from time to time as increasing compression is applied, the user can know that the desired, predetermined compression has been reached when the outer ring member can no longer be manually rotated.
Because the deformation of the inner rings of known, preload indicating washer assemblies is plastic and permanent, these assemblies cannot be reused to indicate proper bolt or fastener loading. Another washer assembly, or at least another inner ring component, must be used. Additionally, if the joint or bolt loses tension due to relaxation etc. of the assembled structure, the act of simply retightening the nut (and thereby re-applying increased compression to the previously-deformed inner ring) will not ensure an adequate joint load.
Thus it is desirable to provide an improved preload indicating washer assembly that is repeatable, i.e. that can be reused and/or re-tightened while retaining the ability to accurately indicate the design or rated compression experienced by it.